This invention relates to infrared ray thickness measuring devices, and more particularly, to devices for continuously measuring with high accuracy very thin transparent to semi-transparent plastic films and films with a high degree of surface planeness.
Very thin transparent or semi-transparent plastic films and films having a high degree of surface planeness are generally produced as a continuous sheet. A film thickness gauge is often used to measure the thickness continuously or at periodic intervals to assure a high quality final product. Two types of film thickness gauges have been used to measure the thickness of the film, contact and non-contact types. In a contact type gauge, a dial or a micrometer in direct contact with the film measures the thickness. In a non-contact type, radioactive rays such as beta or gamma rays are applied to penetrate the film. The amount of radioactive rays absorbed by the film is used to determine the thickness of the film.
A significant drawback to the radioactive non-contact type film measuring device lies in the cumbersome and expensive measures required to protect the operator against undue exposure to the radioisotopes. Naturally, the shielding against the radiation significantly increases the cost of the equipment.
Non-contact type thickness gauges have been proposed using infrared radiation to overcome the drawbacks of the radioactive non-contact thickness gauges. These devices are based upon the different absorptions of infrared rays by the film according to the wavelength of the infrared radiation passing through it. Infrared rays with a wavelength of .lambda..sub.R (hereinafter lambda.sub.R), the reference wavelength, and having a small light absorption factor, are alternately radiated against the film with rays having a wavelength of .lambda..sub.M (hereinafter lambda.sub.M), the measurement wavelength, and having a larger light absorption factor. The intensities of the rays measured after they have passed through the film are converted into a common logarithmic ratio which is then used to determine changes in the thickness of the film.
When the thickness distribution over the width of the film must be measured, the source of infrared radiation and its accompanying sensor are mounted on a scanning frame that reciprocates width-wise over the film as the film is being rolled up. Precise alignment between the source of infrared radiation and the sensor is important, as the variations in the radiation received by the sensor affects the measurement of the thickness of the film. Any deviation in the alignment between the source of the radiation and the sensor resulting from a machining error in manufacturing the scanning frame introduces measurement errors. An effective way to control the manufacturing cost of the scanning frame is to reduce the scanning error attributable to the deviation in the light axis. Attempts to eliminate the scanning error with prior art infrared measuring devices have not met with success.
With the prior art infrared sensing devices, as the film thickness becomes very thin (less than 50 microns (.mu.)) (hereinafter the term "mu" will be used), interference results between the infrared rays reflected by the front and back surfaces of the film. The interference increases the measurement error. One proposal to deal with the interference error has been to use diffused infrared rays rather than parallel rays. The use of diffused rays, however, introduces a significant problem of a large zero point shift resulting from the axis deviation between the projector and the receiver. The diffused rays have not significantly improved measurement accuracy.
Japanese patent application 115850-1976 discloses a means to prevent the interference of infrared measuring devices by radiating the diffused rays onto the material to be measured. The light rays are projected onto a light scattering plate such as frosted glass, with the resulting scattered light rays being used to irradiate the film. Although the measurement accuracy is enhanced by eliminating the interference of the irradiated light with respect to the smooth film surfaces and relatively rough film surfaces, this means is effective only for film thicknesses down to 100 mu. Other devices have been proposed using a high powered infrared radiation source, but they have been low in efficiency and do not meet industry demands.